The present invention relates to an amplifier design, and more particularly, to a multi-stage amplifier circuit with at least a zero and a pole inserted by compensation circuits.
The performance obtainable from a single-stage amplifier is often insufficient for many applications. Therefore, a multi-stage amplifier may be used to achieve the desired performance by cascading several amplifier stages. Taking a three-stage amplifier for example, an output of the first-stage amplifier is used as an input of the second-stage amplifier, and an output of the second-stage amplifier is used as an input of the third-stage amplifier. To suppress the thermal noise, the first-stage amplifier with large transconductance may be employed. However, the unity-gain bandwidth/unity-gain frequency is positively correlated with the transconductance of the first-stage amplifier. In other words, the larger is the transconductance of the first-stage amplifier, the unity-gain frequency is higher and the unity-gain bandwidth is larger. Frequencies at which two high-frequency non-dominant poles of the three-stage amplifier are located may be lower than the unity-gain frequency under a condition that the first-stage amplifier is configured to have large transconductance. Consequently, the three-stage amplifier becomes unstable.
The unity-gain bandwidth/unity-gain frequency is negatively correlated with the Miller capacitance. To address the stability issue, one solution is to increase the Miller capacitance, thereby lowering the unity-gain frequency and reducing the unity-gain bandwidth to achieve stability improvement. However, the dominant pole is also negatively correlated with the Miller capacitance. As a result, the dominant pole is moved to a lower frequency, thus resulting in in-band gain degradation. Consequently, the three-stage amplifier with large Miller capacitance may have poor in-band signal quality.
Thus, there is a need for an innovative frequency compensation design which is capable of enhancing stability of a multi-stage amplifier circuit without degrading the in-band gain of the multi-stage amplifier circuit.